There was a long gap between theoretical physicists working out black holes should exist and finding any, so Cygnus X-1's confirmation caused great excitement. Now that thrill has returned, along with a little puzzlement, with the discovery that our first black hole is more massive than previously realized. So massive, in fact, it casts doubt on what we thought we knew about stellar evolution.
Cygnus X-1 is such a powerful X-ray source that it was discovered during a brief 1964 rocket flight to get a hint of the universe at wavelengths the atmosphere blocks. Ten years later, Stephen Hawking bet against its black hole status, taking 16 years to concede.
Professor Ilya Mandel of Monash University used the Very Long Baseline Array to measure Cygnus X-1's apparent movement compared to distant galaxies, as the Earth swaps from one side of the Sun to the other. In Science, they report these parallax measurements show Cygnus X-1 is 7,100 light-years away, substantially further than previous estimates.
"The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago," Mandel said in a statement. "Incredibly, it's orbiting its companion star – a supergiant – every five and a half days at just one-fifth of the distance between the Earth and the Sun.”
Using what the new distance tells us about the system's dimensions, Cygnus X-1 has a mass around 21 times that of the Sun – 40 percent more than previously thought. Although tiny compared to the supermassive black holes at the center of galaxies, this makes it easily the most massive black hole we have found in the Milky Way resulting from the death of a star.
Black holes are remnants of supergiant stars that became supernovas. Although the largest stars can be born with more than 100 solar masses, their hyperactive solar winds blow most of this off. Along with what is lost in the actual explosion, the black holes we can see with these origins range from 2 to 16 solar masses, so Cygnus X-1 was already near the top of the range. Now it's an outlier.
LIGO has detected gravitational waves from mergers involving black holes up to 50 solar masses, but these are probably the product of stars with very low iron composition, which means weaker solar winds and more mass left behind. Stars recently formed within the Milky Way have higher metal contents, which was thought to therefore mean winds that are too strong to leave so much mass behind.
“Winds get stronger the more massive a star is,” Mandel told IFLscience. “So the black hole's mass doesn't gain much from increasing the initial stellar mass.”
Theoretically, we could be seeing the remains of a triple star system, with the black hole having grown so large because it consumed the third star. Mandel told IFLScience the presence of an additional star with enough mass to make a difference elongates the orbit between the other components, which would usually last after its consumption. Something very abnormal would have been required for a previous triple star system to leave behind a pair with orbits as circular as this one, Mandel said, adding; “Not that anything involving really massive stars is ever normal.”
The most likely explanation then, set out in a companion paper in The Astrophysical Journal, is that we have been overestimating the mass loss of stars like this. Mandel told IFLScience some theoreticians have recently argued previous estimates were too high and “We now have a nice observational constraint on that.”